I had this happen when I had my albino tiger oscar. It's because there are varying levels of albinism.
In order to find a 'clean' albino oscar, you must purchase them around 3-4" and look at their pelvic fins. Any hint of grey, black, or clear, means that that oscar will most likely become a darker, 'dirty', albino.
Look for pelvic fins to be solid white and clean.
HTH,
Serg
BTW, contrary to popular advice, a 55 really isn't good enough for an adult oscar. Just make sure the tank is at least 4-5 feet long, and 2 feet wide.